The present invention relates to a printhead for an image-forming apparatus, containing a substrate, a row of light-emitting elements disposed on a first side of the substrate, and a cooling element disposed on a second side of the substrate opposite the first side. The present invention also relates to an image-forming apparatus provided with such a printhead.
A printhead and apparatus of this kind are known from U.S. Pat. No. 4,703,334. The known printhead is constructed from a ceramic substrate on which a row (array) of light-emitting diodes (LED""s) is disposed. On the first side where the LED""s are located, the printhead is also provided with an image-forming element provided with a selfoc lens array. At the back of the substrate, i.e. the second side remote from the LED""s, there is a cooling element. The latter is constructed as a support plate made from a material having a high thermal capacity, for example aluminium, so that this element can serve as a heat sink to absorb heat. The cooling element is provided with a number of projecting longitudinal ribs which serve to enable the absorbed heat to be transferred to an air flow taken along the ribs. When the printhead is printing, the LED""s produce relatively considerable heat. This heat must be dissipated because the LED temperature must not be too high. A high LED temperature results in a drop in light emission and changes in the wavelength of the emitted light. In addition, the life of the LED""s falls off if they are kept at a high temperature. In the known printhead, the heat generated by the LED""s is discharged via the thermally conductive ceramic substrate to the cooling element which is in turn cooled by a forced air flow. In this way it is possible to prevent the LED temperature from becoming too high during the operation of the printhead so that the optical image-forming characteristics of the printhead remain constant as far as possible. In addition, the low operating temperature means that the printhead life is also sufficiently long.
A printhead of this kind is also known from German patent 38 22 890. Here again, the printhead is constructed around a thermally conductive substrate, in this case a body made from solid copper. The cooling element is constructed from a large number of rod-shaped elements made from a material having a high thermal capacity and conduction. These rod-shaped elements in turn give up the absorbed heat to an air flow which is conducted along the rod-shaped elements by means of a fan.
The known printheads have a number of significant disadvantages. The thermally conductive substrates required to be able to discharge the relatively considerable quantities of heat to the cooling element are speciality products which are expensive, difficult to obtain and often difficult to machine. For example, it is very difficult using such substrates to make structures having a number of layers and mutual connections. Also, the known materials are often brittle or have little shape stability, which further makes printhead production difficult. All this means that the known printheads are expensive to produce, so that the printhead also has a relatively considerable influence on the total production costs of the image-forming apparatus.
A subsequent disadvantage of the known printheads is that the heat produced by the light-emitting elements is discharged uncontrollably as a result of the very intensive but uncontrollable heat discharge via the conductive substrate. One of the results of this is that the array of light-emitting elements may have too great a spread in temperature and hence also in light yield. For example, if the temperature is locally lower than nominal, so that the light yield there is too high, a visible print artefact may form, such as the disappearance of thin lines. Another disadvantage is that the uncontrolled heat discharge always results in uncertainty concerning the form of the substrate (which is temperature dependent) and hence the print characteristic of the print head. A small deformation can in fact, result in defocusing of an LED so that it is no longer possible to obtain sharp illumination of the photoconductor. This has an adverse effect on print quality.
The object of the present invention is to provide a printhead which is inexpensive, for example made from relatively standard materials and with relatively standard processes, and with which it is possible to obtain good and controllable cooling of the light-emitting elements. To this end, a printhead has been developed wherein the substrate is thermally insulating and is provided with at least one thermally conductive track which extends through the substrate from the first side to the second side and is disposed at a predetermined location with respect to the light-emitting elements in order to conduct heat from the first side to the second side in such manner that the elements are maintained substantially at a predetermined temperature during operation of the printhead.
According to the present invention, it is possible to use cheap standard materials as the substrate, for example a glass fiber reinforced epoxy plate. A material of this kind is thermally insulating, but this does not mean that overall, no heat can be dissipated by this material, but rather that the coefficient of thermal conduction is so small that when this material is used the temperature of the light-emitting elements might rise to an unacceptably high level if further steps were not taken with respect to cooling. According to the present invention, the provision of one or more thermally conductive tracks through the material at predetermined locations enables sufficient heat to be discharged from the environment of the light-emitting elements to the cooling element. At the same time, a correct choice of the location where these tracks are provided enables the heat dissipation to be accurately controlled. In this way it is possible not only to prevent the temperature of the light-emitting elements from reaching a specific top limit, but also the temperature of the light-emitting elements can be maintained substantially at a predetermined temperature so that adequate uniformity in the temperature is ensured. As a result, the light emission of the elements will also be sufficiently uniform over the length of the array and the substrate will acquire a form known in advance. The predetermined temperature of the light-emitting elements is typically 30-60xc2x0 C. but, depending on the application, instantaneous load, type of LED""s, wear, and so on, can also be outside that range. In addition, this does not have to be a fixed value but can be adjusted in dependence on the above and other factors so that good print quality can be obtained under all conditions.
Thus using a printhead according to the present invention it is possible to obtain an image-forming apparatus with which it is possible to produce images with a very high print quality and wherein the long life of the printhead helps to reduce service costs. In addition, using the printhead according to the present invention enables the printhead costs themselves to have a reduced influence on the total production costs of the image-forming apparatus.
A printhead is also known from U.S. Pat. No. 5,113,232 which is provided with a row of light-emitting elements disposed on a thermally insulating substrate. In this printhead, the heat is discharged via a conductive metal layer disposed over an appreciable part of the surface of the substrate. In this way, the heat produced by the LED""s is discharged via lateral transport to a heat sink which in this way acts as a cooling element. A construction of this kind has the significant disadvantage that the heat-dissipating power is relatively small, because the heat has to be transported over a relatively large distance by a thin layer. As a result, the temperature of the LED""s can rise to relatively high values. In addition, the substrate itself is heated very non-homogeneously by this construction (only the surface is substantially heated), and this means that during printing the substrate has a considerable risk of becoming deformed due to the occurrence of mechanical stresses in the substrate as a result of an uneven expansion/contraction thereof. A distortion of this kind results in a change of the position of the light-emitting elements, so that the print characteristic of the printhead changes. This takes effect, for example, in a visible deformation of the characters printed with such a printhead. Another disadvantage of this known printhead is that placing further electrical components on the substrate is in conflict with the requirement of adequate lateral heat transport. The electrical connections, in particular, those which are required to actuate these components, cause interruptions in the thermally conductive layer so that the heat dissipation is further limited.
In one embodiment of the printhead according to the present invention, the temperature of the light-emitting elements has a spread over the length of the row such that the light emission over that length has a maximum spread of approximately 15%. By the use of one or more thermally conductive tracks at a predetermined location, heat can be selectively discharged so that a printhead is obtained where the temperature of the light-emitting elements is spread over the row at a sufficiently low value and is also uniform, i.e. lies in an acceptably narrowly limited area. If, for example, a hot spot is systematically present in the row of light-emitting elements, for example because one or more elements are used as outline illumination (which is practically always on), it is possible to discharge more heat locally, for example by the use of a higher concentration of thermally conductive tracks. In this way, a printhead is obtained which has a uniform print characteristic.
In a further embodiment, the row of light-emitting elements is cooled in such a manner that the said temperature has over the length of the row, a spread such that the light emission over that length in turn has a spread of about 10% maximum. This is necessary in environments where an even higher print quality is required, for example in an office environment where a considerable amount of graphic information must be printed. If still higher quality is required, for example if photographs have to be printed, the controlled cooling is preferably such that the temperature difference over the length of the row of light-emitting elements has a spread such that the spread in light emission over that length is about 5% maximum.
In one embodiment of the present invention, the substrate is provided with a thermally conductive layer on the first side, between the light-emitting elements and the substrate. In this embodiment, the heat produced by the row of light-emitting elements is first spread over the substrate in the size of the surface of the thermally conductive layer. This has the advantage that fewer tracks are necessary and the location of the tracks is less critical. In this way, greater degrees of freedom are obtained in the design of the printhead, so that the production costs thereof can be further reduced. In addition, a layer of this kind, if it is also electrically conductive, can serve as a functional electrical contact for the light-emitting elements and possibly other components located on the substrate. It would be possible, for example, to make a layer of this kind in the form of a (semi-)continuous copper film of a specific thickness, typically 35 xcexcm, which layer can simply be applied with standard processes such as are adequately known from the prior art (e.g. electroplating, chemical deposition, gluing, pressure fixing), and so on. A layer of this kind could also be in the form of a set of partial layers, for example thermally conductive rings around a track or in any other way. The characteristic of a layer of this kind is always that heat is transported laterally in the direction of one or more tracks.
In a further embodiment, the thermally conductive track is disposed laterally of the light-emitting elements. In this embodiment, the track, or a plurality of tracks, is not disposed at the location of the light-emitting elements themselves, i.e. in that part of the substrate above which the light-emitting elements are located, but laterally of said elements. In this embodiment, therefore, the tracks are not covered by the LED chip. It has been found that in this way it is possible to make printheads with a more constant print characteristic. This is probably due to the fact that in the case of optical components the accuracy of positioning is of very great importance. Evidently the tracks result in some irregularity at the surface. If the light-emitting components are then placed at the location of said tracks, this results in inaccuracy in positioning which, in the case of a printhead, can result in visible print artefacts. For non-optical components or optical components not used for forming images, such mis-positioning is irrelevant to the functioning of the components. However, it is of maximum importance for printheads of image-forming apparatus. In this embodiment of the present invention, accurate positioning of the light-emitting elements can be obtained at all times. It has also been found that the provision of the tracks next to the light-emitting elements in turn has a favorable effect on keeping the light-emitting elements at the correct operating temperature, so that the uniformity of the temperature over the row of light-emitting elements, and hence the spread in light emission, can in this embodiment be readily controlled to a functionally adequate level, i.e. the spread in light-emission is sufficiently small.
In one embodiment, the track comprises a hollow cylinder in the substrate, the wall of said cylinder comprising a thermally conductive material. A track of this kind differs from a track in which the conduction takes place through a solid element. A hollow track according to this embodiment can be formed easily by drilling a hole in the substrate, typically with a diameter of 0.1 to 0.6 mm, and providing this hole with a conductive metal layer, for example by electroplating, for example copper in a thickness of typically 10-50 xcexcm. Tracks of this kind can easily be made with existing techniques, thus further reducing the cost of a printhead according to the invention. Also, as far as the conductive action of the tracks is concerned, it is of little importance what thermally conductive material is used, and it can, for example, be a metal, or alternatively a ceramic or synthetic material, a mixture of materials, for example conductive metal fibers in a substantially insulating filling agent, and so on. An important feature is that the thermally conductive capacity should be within specific operative limits. These limits depend, inter alia, on the type of light-emitting element, the power generated during printing, the configuration of the printhead, the environment (for example the temperature, presence of natural convection, and so on), the number of tracks, and so on.
In one embodiment, in which the substrate comprises on the first side a driver element operatively connected to the said row for actuating the light-emitting elements, the substrate is provided with at least one additional thermally conductive track at the location of the driver element. In this way, heat produced by the driver element can be directly conducted to the cooling element. In this embodiment, at least one driver (driver chip) is located on the substrate next to the light-emitting elements and serves to actuate the light-emitting elements. It can, for example, be a loose chip or alternatively a chip integrated with the chip containing the light-emitting elements. For the driver itself, a uniform and low temperature is of itself of less importance, but since in this embodiment the driver is located on the same substrate it is important that the temperature of this driver also should not be too high or too low and in addition should not differ too much from the temperature of the light-emitting elements. Otherwise, for example, mechanical stresses might form in the substrate and be sufficient to result in distortion of the substrate. As already indicated hereinbefore, such distortion can give rise to print artefacts. Also, an excessive driver temperature can result in heating of the light-emitting elements, and this is undesirable as will be apparent from the foregoing.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.